Preliminary evidence suggests that ambrisentan is safe and effective in patients with portopulmonary hypertension. The goal of therapy for these patients is to improve symptoms of dyspnea and to improve pulmonary hemodynamics to a mean pulmonary artery pressure <35 mm Hg in order to make patients eligible for liver transplantation. Therefore, the primary endpoints for this study will include 6 minute walk distance (6MWD) and pulmonary vascular resistance (PVR).